Tet system for implanted medical device

ABSTRACT

A TET system is operable to vary an amount of power transmitted from an external power supply to an implantable power unit in accordance with a monitored condition of the implantable power unit. The amount of power supplied to the implantable power unit for operating a pump, for example, can be varied in accordance with a cardiac cycle, so as to maintain the monitored condition in the power circuit within a desired range throughout the cardiac cycle.

This application claims priority of U.S. Provisional Application No.61/191,595, filed Sep. 10, 2008, the entire content of which is herebyincorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to a transcutaneous energy transfer (TET)system and a TET system method of operation.

Transcutaneous energy transfer (TET) systems are used to supply power todevices such as heart pumps implanted internally within a human body. Anelectromagnetic field generated by a transmitting coil outside the bodycan transmit power across a cutaneous (skin) barrier to a magneticreceiving coil implanted within the body. The receiving coil can thentransfer the received power to the implanted heart pump or otherinternal device and to one or more batteries implanted within the bodyto charge the battery.

One of the challenges of such systems is insufficient battery lifetime.The implanted battery may be required to supply the implanted device'sentire power demand for one to several hours at a time, such as when thepatient does activities that preclude wearing the external TET powerunit, such as showering or swimming. When the implanted battery is firstimplanted into the patient, the battery capacity is large and can meetthe power demand for the required amount of time. However, whensubjected to frequent charging and discharging, the implanted battery'scapacity decreases. With decreased battery capacity, the patient cannotspend as much time without the external TET power unit. Eventually, thebattery may need to be replaced so that the patient can go without theexternal TET power unit for long enough periods of time again.

Until now, premature wear-out of the implanted battery due to frequentcharging and discharging of the battery was believed to be unavoidable.Conventional TET systems do not supply power closely in accordance withthe time-varying power requirements of implanted devices. As a result,when the implanted device has rapidly fluctuating power demands such ascharacteristic of circulatory assist pumps including left ventricleassist devices (“LVADs”), the implanted battery is required to supplypower for momentary high power demands and the TET system recharges thebattery when the momentary power demands ease.

SUMMARY OF THE INVENTION

In accordance with an aspect of the invention, a TET system is providedfor powering an implanted electrical device. In a particular example, acirculatory assist device can be provided which can include one or moreelectrical devices such as a pump having an electric motor, the pumphaving a power demand which varies with the cardiac cycle of a patientin which the pump is implanted.

An implantable power unit is adapted for mounting within the body of thepatient. The power unit may have a secondary coil and a power circuitconnected to the secondary coil for controlling and supplying power tocontrolling circuitry and to the pump, for example. In that way, powercan be received at the secondary coil and applied to controllingcircuitry and to the pump. In one embodiment, a monitoring circuit ofthe implantable power unit can be used to monitor a condition of thepower circuit. The monitoring circuit can transmit a transcutaneoustelemetry signal which represents the monitored condition for use by acontrol circuit of an external unit to adjust power transmission.

An external power supply may be adapted for transcutaneous inductivecoupling with the secondary coil, and may have a drive circuit operableto apply an alternating current to the primary coil, and a controlcircuit operable to receive the telemetry signal and adjust thealternating current in the primary coil at least in part in response tothe telemetry signal. In a particular embodiment, the monitoring circuitand control circuit may be operable to monitor the condition of thepower circuit and adjust the alternating current in the primary coil inaccordance therewith. In that way, the alternating current in theprimary coil may be varied substantially in accordance with the cardiaccycle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cutaway sectional diagram illustrating componentsand operation of a TET system in accordance with an embodiment of theinvention.

FIG. 2 is a block and schematic diagram further illustrating externaland internal components of a TET system in accordance with an embodimentof the invention.

FIG. 3 is a block and schematic diagram further illustrating componentsof an external module of a TET system in accordance with an embodimentof the invention.

FIG. 4 is a block and schematic diagram further illustrating componentsof an implanted module of a TET system in accordance with an embodimentof the invention.

FIG. 5 is a schematic diagram illustrating components of bridgerectifier circuitry of an implanted module of a TET system in accordancewith an embodiment of the invention.

FIGS. 6A and 6B are perspective drawings illustrating placement andrelationship of components of an implanted TET module viewed from anexterior surface in accordance with an embodiment of the invention.

FIG. 7 is a graph illustrating a rate of feedback sampling andtransmission of power between external and implanted modules of a TETsystem in a method of operation in accordance with an embodiment of theinvention.

FIG. 8 is a flowchart illustrating a method of operation in accordancewith an embodiment of the invention.

FIGS. 9A-C illustrate operation in accordance with a particularembodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a transcutaneous energy transfer (TET)system 100 used to supply power to an implanted therapeutic electricaldevice 102 in an internal cavity within the body, i.e., below the skinof a patient 104. The implanted electrical device 102 can include a pumpsuch as for use in pumping blood as a ventricular assist device (“VAD”),for example. The implanted electrical device 102 can include controllingcircuitry to control, for example, a pump.

As depicted in FIG. 1, the TET system 100 includes an external module110 having a primary power coil 114, associated circuitry 116 andterminals 111 for receiving an external source 112 of power. An internalmodule 120 implanted underneath the skin of the patient 104 has asecondary power coil 124, associated circuitry 126 and an output cablefor supplying power to the implanted electrical device 102. Power istransferred from the primary coil 114 to the secondary coil 124 by meansof inductive coupling, i.e., via near-field interaction of a magneticfield overlapping the primary 114 and secondary 124 coils. The voltageacross each coil can be large, for example, peak-to-peak voltages of 100V to 400 V are not uncommon. The implanted module 120 is also connectedto an implanted battery 128 for supplying power to the implantedelectrical device 102 in case power to the external module 110 whenpower transmission is interrupted between the external 110 and implanted120 modules. With the implanted battery 128 as a backup, the externalTET module 110 can be disconnected when the patient bathes or performsother activities.

FIG. 2 is a functional block diagram illustrating electrical componentsof the TET system 100. As illustrated therein, the external module 110of the TET system 100 includes the primary coil 114 and associatedcircuitry including a microcontroller 212, a radio frequency (“RF”)telemetry system 214 and a TET driver 216. To reduce losses due to skineffect, the primary coil 114 can be fabricated using Litz wire, in whichthe primary coil 114 is made up of relatively thin, insulated wirestwisted or woven together in groups. Power transfer from an externalpower source 112 to the implanted module 120 is provided through the TETdriver 216 as controlled by microcontroller 212.

The implanted module 120 includes a TET receiver 226 including thesecondary coil 124, a microcontroller 222 and an RF telemetry system224. Like the primary coil 114, the secondary coil 124 can also befabricated using Litz wire. The TET receiver 226 includes rectifiercircuitry, such as a diode bridge, for converting electrical energy atthe secondary coil in alternating current (“AC”) form into directcurrent (“DC”) form. DC power output from the TET receiver 226 issupplied to a microcontroller 222 of the implanted module 120, animplanted battery 128 and an implanted electrical device 102. Theimplanted electrical device 102 can include one or more of a variety ofdevices such as a VAD blood pump, for example, which has power demandswhich could not be supplied by the implanted battery 128 for longperiods of time. In such case, the implanted battery 128 is not aprimary power source, but is used to supply power for relatively shortperiods of time in case of an interruption in the transmission of powerto the implanted module 120. For example, the implanted module 120 canrely on battery power when the patient takes a shower.

FIG. 3 is a block and schematic diagram illustrating operationalcomponents of the external module 110 in greater detail. As illustratedtherein, a power management module 314 under control of microcontroller212 transfers power in DC form to a variable output level power supply316 from one or more external power sources 312 a, 312 b. The externalpower sources 312 a, 312 b can include one or more batteries, or onebattery and an external AC/DC converter coupled to an AC source (such asa wall outlet) or a DC source, such as from within an automobile, forexample.

The power management module 314 regulates the flow of power from the oneor more external power sources 312 a, 312 b to a variable output powersupply 316. This power module 314 has terminals 334 a for connectionwith a first external power source 312 a and has terminals 334 b forconnection with a second external power source 312 b. The power module314 may have more sets of terminals (not shown) for connection withpower sources (not shown) other than sources 312 a, 312 b. The powermanagement module 314 can determine which particular sources or types ofsources are connected thereto and may also detect to which sets 334 a,334 b of terminals the power sources 312 a, 312 b are connected. Module314 determines whether or not power sources are connected thereto andmay also determine the status of each connected power source, i.e., thevoltages of each power source and the charge state of battery powersources. The power management module 314 also selects one or more ofseveral connected power sources to draw power from in supplying energyto drive the TET system 100. For example, when both a battery and an ACor DC power source other than a battery are connected, power module 314may use the AC or DC power as a primary source to power the external TET110 and hold the battery in reserve for use in case the primary sourcebecomes disconnected. The power management module 314 can also be usedto regulate the flow of a charging current to one of the external powersources 312 a, 312 b, such as when the second power source 312 b is abattery.

The variable power supply 316 provides power to a TET driver 318 at arate which is subject to vary in accordance with the time-varying needfor power of the electrical implanted therapeutic device 102 (FIG. 2).The power transfer rate to the TET driver 318 can be varied by modifyingthe voltage Vs at which power is output by the power supply 316 undercontrol of one or more signals output by microcontroller 212. In oneexample, the output voltage Vs can be varied between 13 V and 25 V, inorder to adjust between varying power demands and supply powerefficiently to the TET driver 318.

The TET driver 318 supplies an excitation current to the primary coil114 for transferring power to the implanted TET module 120. The TETdriver 318 receives power at a steady (DC) supply voltage Vs andgenerates a magnetic flux for power transmission which has an ACwaveform at a relatively low radio frequency (RF). Typically, thefrequency of the AC power transmission waveform is set between about 30kilohertz (kHz) and 300 kilohertz. Power is transmitted by inductivenear-field coupling between the primary coil 114 and the secondary coil124 (FIG. 2) of the implanted module 120.

The primary coil 114 is connected in series with a capacitor 330 in atank circuit 331. The tank circuit 331 resonates at a resonant frequencydetermined by the inductance value of the coil 114 and the capacitancevalue of capacitor 330. The TET driver 318 includes a set of power-ratedfield effect transistors in an H-bridge arrangement, e.g., MOSFETs,which drive the primary coil 114 in a push-push fashion under control oflogic drive circuits.

The TET driver 318 can regulate the transfer of power between theprimary coil 114 and the secondary coil 124 (FIG. 2) in three ways. TheTET driver 216 can output an excitation current to the primary coil 114in a pulsed manner and vary the width of the drive pulses supplied tothe coil 114 and hence, the duty cycle of such pulses. The TET driver216 can also vary the frequency at which drive pulses are supplied tothe primary coil 114 to create a desired balance between efficiency ofpower transfer throughout the system and stability of power regulationin the system. In addition, as mentioned above, the supply voltage Vs atwhich power is provided to the H-bridge circuit can be varied.

As further shown in FIG. 3, the external module 110 can include athermistor 332 situated in the vicinity of the primary coil 114 fordetecting a temperature of the coil 114 and providing a signalrepresentative of the temperature to microcontroller 320. If thetemperature increases to an excessive level which is uncomfortable orunsafe to the patient, the microcontroller 212 can alert the patientabout possible coil misalignment. To allow the temperature to return toa more normal level, when possible, the microcontroller 212 cantemporarily alter the operating mode of the implanted module 120 toreduce power requirements, such as by suspending charging of theimplanted battery 128.

As further illustrated in FIG. 3, the external module 110 includes anover-voltage protection (OVP) circuit 322 having an input coupled to anamplifier 324 to receive a signal representative of a voltage Vc acrossthe primary coil 114. OVP 322 detects when Vc reaches an excessive leveland provides a signal to TET driver 318 to shut off the TET driver 318until Vc reaches a safe level again. In one embodiment, OVP 322 is ahardware-controlled circuit, i.e., one which responds to theover-voltage condition on its own through operation of internalelectrical circuitry, which may include hard-wired logic circuits.Therefore, OVP 322 responds rapidly to an over-voltage condition withoutrequiring software instructions to be retrieved and executed within OVP322. This circuit typically also does not need to await signals orinstructions from microcontroller 212 to respond.

The external module 110 further includes a bi-directional radiofrequency (RF) telemetry block 214 having a bi-directional signalinterface with the microcontroller 212. The telemetry block 214 isarranged to transmit and receive signals through a transceiver andattached antenna 340. The telemetry block 214 is arranged to acceptsignals output by the microcontroller 212 for controlling operation ofthe implanted module 120, as will be described further below. Thetelemetry block 214 also receives various signals transmitted by acorresponding transceiver 224 (FIG. 2) of the implanted module 120 formonitoring operation of the implanted module 120.

The microcontroller 212 uses information received through the linkbetween the telemetry modules 224, 214 of the implanted module 120 (FIG.2) and the external module 110, as well as a signal 342 representativeof supply current, a signal 344 representative of the supply voltage Vsand a signal 346 representative of the coil voltage Vc in order tovariably supply energy in an efficient manner to the primary coil 114and to address safety concerns.

FIG. 4 is a block and schematic diagram further illustrating componentsof the implanted module 120 in accordance with an embodiment of theinvention. As mentioned above, the implanted module 120 has amicrocontroller 222 for controlling its operation, particularly withrespect to the operation of the TET power receiver 226 through whichpower is received from the external module 110. The implanted module 120also has an RF telemetry block 224 and a backup telemetry block 402. TheRF telemetry block 224 includes a radio frequency transceiver used totransmit signals representative of measurements of operationalparameters of the implanted module 120 and to receive signals from theexternal module 110 for controlling operation of the implanted module120. A backup telemetry module 402 having a resonant tank circuit 404including a capacitor 405 and an inductive data coil 406 separate fromthe secondary coil 124 is available to transmit the signals representingmeasured parameters from the implanted module 120 to the external module110. Thus, the backup telemetry module 402 can transmit signals to thebackup telemetry receiver 350 in the external module 110 (FIG. 3) byinductive coupling between the coil 406 and the primary coil 114 of theexternal module 110. In a particular embodiment, the backup telemetrymodule 402 can transmit signals in an inductively coupled manner to thebackup telemetry receiver 350 in accordance with technology as specifiedin ISO 14443 or a further development thereof known as “Near FieldCommunication” (“NFC”) as specified by the NFC Forum, Inc. Suchtechnologies can be advantageous for transmitting signals shortdistances (e.g., less than centimeters) at relatively low frequencies.

The resonant frequency and size of this inductive data coil 406 can beset for efficient operation at a frequency between the relatively lowfrequency (typically below 300 kHz) used for power transmission by wayof coils 114, 124 and a standard transmission frequency (approximately400 MHz) at which signals are normally transmitted between the RFtelemetry transceivers 224, 214 (FIG. 3). In one example, thetransmission frequency of the backup telemetry module 402 is about 10megahertz (MHz). In one example, the backup telemetry module 402 may becapable to transmitting signals at a frequency designated for Near FieldCommunication of 13.56 megaHertz.

The backup telemetry module 402 need not be always active. In oneembodiment, the backup telemetry module 402 is active only when thebi-directional RF telemetry system is unavailable or ineffective incommunicating measurements of operational parameters between themicrocontroller 222 of the implanted module 120 and the microcontroller212 (FIG. 3) of the external module 110. For example, the backuptelemetry module 402 can be utilized when signals representingup-to-date operational parameters for the implanted module 120 are notreaching the microcontroller 212, such as due to malfunction of the RFtelemetry block 224 or due to interference in transmitting outboundsignals over the primary link between RF telemetry blocks 224 and 214(FIG. 3). In this context, interference can occur as “out-of-band”interference in which noise and emissions are present on the airwaveswhich make receiving signals from the RF telemetry block 224 moredifficult. In-band interference can also occur, in which several similardevices are competing to transmit their signals over the same bandwidth.In one example, multiple transmitters can be operating at the same timeand place in implanted devices of patients such as in an intensive careunit. For example, the implanted medical devices can transmit signalsover a designated frequency band in accordance with the standard“Medical Implant Communications Service (MICS).” Interference can occurwhen too many devices are competing to use the allocated bandwidth atthe same time. When devices are competing to use the same bandwidth, theamount of time each device can utilize the bandwidth may be reduced. Insuch case, when data loads to be transmitted from the implanted module120 to the external module 110 are too great for the amount of timeallocated on the bandwidth, it becomes difficult to transmit the signalsrepresenting the operational parameters for the implanted module 120.

When significant interference is present, the backup telemetry module402 can be utilized in place of the regular RF telemetry block 114 totransmit data from the implanted module 110. When the interference isover, operation of the backup telemetry module 402 can be halted andtransmission can resume from the primary RF telemetry block 214.

In one embodiment, the backup telemetry module 402, when activecontinuously and repetitively transmits the present valid powerregulation data. The external TET controller 212 temporarily interruptsthe power transmission drive pulses to TET driver 216. During this pausethe backup telemetry receiver 350 of FIG. 3 listens for the low levelbackup telemetry signals. In this way, the backup telemetry receiverblock 350 can receive low-power (millivolt level) backup telemetrysignals in intervals between power transmission drive pulses whichnormally produce large wide band noise transients in the primary 114 andsecondary 124 coils.

Transmission of the operational measurement signals by the backuptelemetry module 402 can be performed by causing the resonant (carrier)frequency of the tank circuit 404 to be pulsed in a digitally encodedmanner. For example, the carrier frequency of the tank circuit 404 canbe switched on and off in succession using a pulse position modulationscheme. Stated another way, the carrier frequency of the tank circuit404 can be pulsed, then paused for a relatively short period of time,then pulsed again to transmit a symbol indicating a digital value suchas “0”. In such example, the carrier frequency can also be pulsed, thenpaused for a longer period of time and then pulsed again to indicate adifferent type of symbol, such as “1” in a binary signalling scheme of“1's” and “0's”, for example. The number of transmitted pulses and thenumber and placement of shorter and longer pauses between them can beused to represent different transmitted symbols.

Referring to FIG. 4, the TET receiver 226 outputs a supply voltage Vteton line 412 to a power manager 422. The supply voltage Vtet is avariable voltage which can be controlled in order to vary the amount ofpower (i.e., current) being supplied to a motor controller or to theimplanted battery 128 under different demand conditions. Themicrocontroller 222 may help to maintain Vtet at a higher voltage whenhigher power demands are present. In such case, as described below, themicrocontroller 222 can transmit a signal back to the external module110 to increase the rate of power transmission and in so doing, causethe voltage Vtet to increase. On the other hand, if the supply voltageVtet is too high, the microcontroller 222 may help to reduce Vtet to alower voltage when lower power demands are present. Again, themicrocontroller 222 can transmit a signal back to the external module110 to reduce the rate of power transmission and cause the voltage Vtetto fall.

The TET receiver 226 includes over-voltage protection circuitry 408activated in case of an excessively high DC voltage level output by theTET receiver 226. The over-voltage protection circuitry 408 may be anover-voltage protection clamp. An excessive DC voltage can occur whenthe rectified AC current greatly exceeds the DC current demand of theimplanted device. Implant power regulation is achieved by controllingthe amount of current delivered to the load (implanted device) whilemaintaining a given voltage range. The drive level to the TET driver 216and the primary coil 114 is modulated to provide the desired loadcurrent to the implanted module 120 at the specified voltage range.While the clamp 408 is active, the telemetry signals to the external TET110 will reduce the drive levels to restore proper current output.Clamping the output voltage prevents damage to the implant system andincrease reliability. Over voltage clamping normally occurs after alarge step decrease in load current such as during pump start-upsequences.

A comparator 410 connected to receive the DC output voltage Vtet candetect when the DC output voltage exceeds a predetermined over-limitthreshold. A signal output by the comparator 410 then activates theover-voltage protection circuitry 408, causing an immediate reduction inthe DC output voltage Vtet. The over-voltage protection circuitry 408remains active until the output voltage Vtet drops below a secondpredetermined threshold which is substantially lower than thepredetermined over-limit threshold. When the lower threshold is crossed,the over-voltage protection circuitry 408 turns off again and thecircuitry 414 resumes outputting the DC voltage Vtet as normal. In oneexample of operation, the supply voltage Vtet may be kept at a level ofaround 16 V. The over-limit threshold may be set to 25 V and the lowerthreshold for resuming operation may be set to 20 V. In such case, theover-voltage protection clamp 408 is intended to operate only from timeto time when Vtet goes well beyond the normal range. One event likely toactivate the over-voltage protection circuitry 408 is a step decrease inthe power demand of the implanted module 120. Such decrease can occur,for example, when the power manager 422 turns off battery charging(because the implanted battery 128 is now fully charged) or the start-updemand of the motor of a high-wattage implanted electrical devicesubsides. Cessation of operation of such device might also cause thevoltage Vtet to spike.

As further shown in FIG. 4, the TET receiver 226 includes a set ofbridge rectifiers and filters 414 for converting the form of the energyreceived at the secondary coil 124 from AC to DC. FIG. 5 is a schematicdrawing showing the bridge rectifier and filter circuitry 414 in greaterdetail. The rectifier incorporates a set of diodes D2, D3, D4 and D5connected in a bridge arrangement to receive the output of the secondarycoil 124 for providing full-wave rectification of the transmitted ACpower waveform. Two additional diodes D1 and D6 are blocking diodes toprevent FET body diode conduction when the clamp FETs Q1, Q2 are off.The gates of transistors Q1 and Q2 are connected through resistor R1 toground, which keeps the transistors turned off when signal 411 isinactive.

All of the diodes shown in FIG. 5 can be Schottky diodes which arewell-suited for rectifier applications, having relatively lowon-resistance and turn-on voltages of about 0.2-0.3 V. Transistors Q1and Q2 typically are n-type metal-oxide semiconductor field effecttransistors (n-type MOSFETs), the sources of the transistors beingconnected to ground. Capacitors C1 and C2 series resonate with secondarycoil 124 at the nominal drive frequency and are of low loss metalizedplastic film construction. The capacitors are of equal value to evenlydistribute the load current and losses to increase reliability.Capacitors can be selected for this purpose which are capable ofhandling relatively high current and voltage and which dissipate onlysmall amounts of energy during normal operation. In one example, inorder to provide operating margins and allow for transients, thecapacitors may need to be capable of handling currents of severalamperes and voltages of 600 V peak to peak or more. The second set ofcapacitors C3 and C4 may also include one electrolytic capacitor andanother film capacitor having a smaller value, and these capacitors canoperate to filter the rectified output voltage Vtet.

FIG. 4 further illustrates a current monitor circuit 420 coupled tobridge rectifier circuitry 414. The current monitor 420 provides inputto the microcontroller 222. Highest current demands are during pumpstart up. When current detected by the current monitor 420 is excessive,the microcontroller 222 can provide a signal to a motor controller 222or power manager 422 to reduce current demands.

Within the implanted module 120, a thermistor 424 may also be providedin close proximity to the housing 620 for detecting the module'stemperature and providing a signal representative of the temperature tothe microcontroller 222. If the temperature of the implanted module 120becomes excessive, the microcontroller 222 can indicate theover-temperature condition to the external module 110 by outputting asignal thereto through the RF telemetry transceiver 224 or the backuptelemetry transceiver 402.

FIGS. 6A and 6B provides perspective diagram depicting an exterior ofthe implanted module 120. The secondary coil 124 can be fabricated inform of an oval or circular ring 610. The bridge rectifier circuitry 414and over voltage protection circuitry 408 can be hermeticallyencapsulated within a housing 620 which presents an exterior surface ofbiocompatible material. The housing 620 may fit within the inner wall612 of the ring 610 as shown in FIG. 6. The housing 620 within the ring610 may include a ceramic material to help distribute the heat generatedby the bridge rectifier circuitry 414 so that temperature changes at theexterior of the implanted module 120 are gradual and limited to a smallamount. An encapsulant 626 extending between the housing 620 and anouter wall 614 of the ring 610 can assist in spreading heat generated bythe primary coil 124 and the bridge rectifier circuitry 414 over agreater volume. Providing the bridge rectifier circuitry 414 and theover voltage protection circuitry 408 inside the inner periphery of thering may help to spread the heat generated by the coil 124 and thebridge rectifier circuitry 414 uniformly over these heat-generatingelements.

In accordance with an embodiment of the invention, a method of operationof the TET system 100 will now be described. For some applications suchas for supplying power to a VAD, the TET system 100 (FIG. 1) is intendedto continually transfer power between the external module 110 and theimplanted module 120; that is, during every second of each hour, everyhour of the day and every day of the year. The flow of electrical powerfrom the external module 110 to the implanted module 120 is preciselymetered according to the instantaneous power demand of the VAD asmonitored by the implanted module 120. Moreover, the amount of powertransferred from the external module 110 to the implanted module 120 canbe continuously adjusted at intervals of less than a second (e.g., from50 times a second to 5000 times a second, for example) such that theimplanted module 120 only receives as much power as it needs during suchinterval to power the implanted VAD or other therapeutic device 102. Inthat way, power is not drawn from the implanted battery 128 duringnormal operation of the VAD.

Stated another way, power is drawn from the implanted battery 128generally only when there is an interruption in the transfer of powerbetween the external 110 and implanted 120 modules or external power isdeliberately disconnected, such as when the patient takes a shower. Aninterruption in power transfer can also occur when there is a grossmovement of the patient that seriously affects coupling between thecoils 114, 124 of the external 110 and implanted 120 modules, or whenswitching external power sources connected to the external module 110.

Thus, the microcontroller 222 rapidly samples (50 to 5000 times asecond, for example) the voltage level Vtet on power supply line 412.The microcontroller 222 then causes RF telemetry transceiver 224 tocommunicate information back to the external module 110 which can beused in the external module 110 to ascertain the voltage level Vtet. Forexample, the microcontroller 222 may cause the transceiver 224 totransmit information in each communication to the external module 110which directly indicates the voltage level Vtet.

Alternatively, the transceiver 224 can be used to transmit informationmerely indicating the voltage is higher or lower than the previouslymonitored value. In the case that transceiver 224 transmits the signalmerely indicating the voltage level Vtet has increased, logic in themicrocontroller 212 of the external module 110 can then determine thecurrent voltage level Vtet by adding a fixed incremental value (e.g.,0.1 V) to the voltage level recorded just prior thereto. Otherwise, inthe case that transceiver 224 transmits the signal merely indicatingthat the voltage level has decreased, logic in the microcontroller 212of the external module 110 can then determine the current voltage levelVtet by subtracting a fixed incremental value (e.g., 0.1 V) from thevoltage level recorded just prior thereto. Such sampling and reportingof results from the implanted module 120 to the external module 110 on acontinuous, rapid and consistent basis allows the microcontroller 212 tomaintain synchronized rapidly updated information indicating thevariable voltage level Vtet in the implanted module 120.

Moreover, having synchronized rapidly updated information concerning thevoltage level Vtet allows the external module 110 to supply power to theimplanted module 120 according to the continuously changing power demandof the VAD or other implanted electrical device. An example of suchoperation can be seen in FIG. 7.

A patient's systemic blood pressure varies with time during the cardiaccycle. For this reason, a patient's blood pressure is usually expressedusing two numbers, such as 115/75, for example. The higher numberrepresents systolic blood pressure at maximum pressure reached becauseof the heartbeat. The lower number represents diastolic blood pressureat minimum pressure between heartbeats. Thus, once during each cardiaccycle the blood pressure rises to the systolic number with eachheartbeat and falls again to the diastolic number before the nextheartbeat.

Blood pressure and the flow of blood through the heart significantly andrapidly affect the load placed on a circulatory pump such as a VAD. Dueto changes in load, the amount of current required for the pump tooperate varies significantly during each cardiac cycle.

Referring to FIG. 7, a peak current (“P”) is required to drive the motorstator of the VAD once during each cardiac cycle of pumping blood fromthe left ventricle into the aorta to pressurize the blood flowing intothe aorta. The peak current is substantially greater than a reducedcurrent value (“R”) which is needed to drive the motor stator of the VADat another time once during each cardiac cycle.

FIG. 7 indicates sampling intervals in which the voltage level Vtet issampled. At a rapid sampling rate of 50 to 5000 samples per second, thevoltage level is sampled 50 or more times per cardiac cycle, i.e., atthe rate of 50 or more times per beat when the heart is beating at arate of once per second, i.e., at 60 beats per minute. Therefore, FIG. 7indicates that the voltage level has been sampled already 200 times bythe time that one cardiac cycle (“T”) has ended.

Because the power demand of the implanted VAD varies greatly (between Pand R) during each cardiac cycle, power needs to be delivered to theimplanted module 120 in a matching way. In order to fulfil thisobjective, the microcontroller 222 of the implanted module 120 samplesthe supply voltage Vtet and causes the RF telemetry transceiver 224 totransmit information back to the external module 110 for monitoring theimplanted module's 120 internal supply voltage. If the internal supplyvoltage Vtet goes down, the implanted module 120 transmits a signal backto the external module 110. The external module 110 then transmitsgreater power to the implanted module 120, causing the internal supplyvoltage to rise towards the normal level. As indicated above, theexternal module 110 can vary power transmission in three ways. First,the external module 110 can vary the width of pulses during which poweris transmitted by the primary coil 114. Secondly, the external module110 might vary the frequency of such pulses, e.g., by a percentage of acarrier frequency depending upon coupling and load conditions. Thirdly,the external module 110 might also vary the power supply voltage Vsinput to the TET driver 318 (FIG. 3). The external module 110 may varythe power transmitted via the primary coil 114 in one, two or all threeof these ways in order to achieve the desired result in the implantedmodule 120. In turn, the implanted module 120 receives an amount ofcurrent (power) which is adjusted on a rapid and real-time basis inresponse to a monitored condition of the power circuit in the implantedmodule 120. In one example, one way that the amount of transmitted powercan be adjusted is for the external module 110 to vary the primary drivepulse width of alternating current applied to the primary coil 114.Small changes in that pulse width can vary the amount of transmittedpower significantly in accordance with the degree of resonance achievedin each of the primary 114 and secondary 124 coils.

On the other hand, if the internal supply voltage Vtet goes up, theimplanted module 120 transmits a signal back to the external module 110.By reducing the primary drive pulse width, the external module 110 cantransmit less power to the implanted module 120, causing the internalsupply voltage to fall back towards a normal level.

Moreover, while the variation in power demand occurs as a function ofthe cardiac cycle, the rate of power transmission may also need to bevaried according to the efficiency of transmission. Specifically,variations in coupling between the primary 114 and secondary 124 coilsdue to movement of the patient or interference, e.g., from objects orliquids (such as while bathing) can raise or lower the efficiency ofpower transmission. The monitored supply voltage information from theimplanted module 120 can be used to raise or lower the powertransmission rate to adjust for these changes in transmission efficiencyrelated to coupling. In addition, whenever a strong change is detectedin the voltage Vs of the external module 110 or Vtet of the implantedmodule 120, this can be a sign that the position of the primary coil 114of the external module 110 has moved and needs adjusting. The externalmodule 110 can then produce an audible, visible or tactile (e.g.,vibrational) signal to the patient to indicate that the primary coil 114positioning needs adjusting.

Whenever there is a problem in feeding back signals relating to theinternal supply voltage Vtet to the external module 110 by way of theprimary telemetry transceivers 224, 214, the backup telemetry system 402can be used to transmit such information. When the backup telemetrysystem 402 is active, receiver 350 (FIG. 3) of the external module 110“listens”, i.e., picks up the signal from the primary coil 114 inintervals between individual power transmission pulses used to transmitpower from the external module 110. In this way, rapid sampling andrapid adjustment of power transmission to the implanted module 120 canbe maintained even when the primary telemetry transceivers 224, 114 areinoperable or transmission bandwidth is unavailable.

If the microcontroller 222 of the implanted module 120 senses anover-current condition via current monitor circuit 420, the telemetrysystem 224 or backup telemetry system 402 can be used to signal theevent back to the external module 110. The rate of power transmission tothe implanted module 120 can then be lowered or temporarily interrupteduntil the over-current condition is no longer present.

In a particular example of operation, the microcontroller 222 of theimplanted module 120 may sense an over-temperature condition via atemperature sensor 424. In order for the implanted module 120 to remaincomfortable to the patient, the temperature rise at the exterior surfaceof the implanted module 120 may need to be contained to one to twodegrees Celsius. To lower the temperature in response to theover-temperature condition, the microcontroller 222 may temporarilyreduce the current demand. For example, the microcontroller 222 cantemporarily reduce the amount of current delivered to the motor or thatis used for charging the implanted battery 128. When changing the amountof current used by the implanted module 120, the supply voltage Vtet maychange. Changes in the supply voltage then are monitored andcommunicated to the external module 110 via the telemetry system 224 orbackup telemetry system 402. The changes can then be addressed in amanner as described above such that the voltage Vtet is brought backwithin normal range.

Using thermistor 332, the external module 110 may also sense anover-temperature condition near the primary coil 114. As indicatedabove, the over-temperature condition may result from excessive currentdraw by the implanted module 120. To address the over-temperaturecondition, the microcontroller 212 of the external module 110 maytransmit a signal to the implanted module 120 using the telemetrytransceiver 214. In that way, the implanted module 120 can temporarilyreduce the amount of current supplied to the motor or to the implantedbattery 128 to cause the temperature to return to normal.

FIG. 8 is a flowchart illustrating a method of operation in accordancewith an embodiment of the invention. In such embodiment, control isaffected over the transmission frequency at which power is transmittedbetween the transmitting primary coil 114 of the external module 110 andthe receiving secondary coil 124 of the implanted module 120. In suchway, the transmission frequency can be maintained at a value which has apredetermined difference with respect to a resonant frequency of thepower transmission system including the transmitting primary 114 andreceiving secondary 124 coils. Thus, the variable transmission frequencycan be adjusted to a value which is “near” (e.g., one to a few percentabove or below) the resonant frequency, but not at the resonantfrequency. Maintaining the transmission system at near resonance helpsaid the stability of the system. When the system operates at theresonant frequency itself, a small change in the degree of coupling,i.e., the mutual inductance between the coils, can produce relativelylarge changes in the voltage across the transmitting primary 114 andreceiving secondary 124 coils. The system may then require frequentadjustments to the voltage or current supplied to the transmittingprimary coil 114 in order to stay within assigned operational limits.

On the other hand, when the system operates at a frequency near theresonant frequency but not at the resonant frequency, the system canoperate with greater stability and power can be transmitted efficientlybecause the system may not need to be adjusted as frequently.

Thus, a parameter related to mutual inductance between the transmittingprimary coil 114 and the receiving secondary coil 124 can be monitored.In one example, the parameter can include an estimate of the distance ordisplacement between the two coils. Using the monitored parameter, thevariable transmission frequency can then be adjusted to a value whichhas a predetermined difference from the resonant frequency. In this way,the transmission frequency can be maintained with a predetermineddifference, e.g., percentage difference, or absolute difference infrequency, from the resonant frequency between the transmitting primary114 and receiving secondary 124 coils for the particular spatialdisplacement between the two coils.

In a particular embodiment (FIG. 8), a voltage across the transmittingprimary coil 114 in the external module 110 is monitored (810). Themeasured voltage is an average voltage taken over a given period oftime, for example 250 msec. This measured voltage helps to filter outvariations in cardiac cycle. An increase in voltage can indicate greaterspatial displacement between the transmitting primary 114 and receivingsecondary 124 coils. For example, the voltage can increase when theexternal module 110 moves away from the implanted module 120, such aswhen the patient moves or bends. From the measured voltage, the distancebetween the two coils can be estimated (820). From the estimate of thedistance, a new frequency value can be determined to which thetransmission frequency should be adjusted. In a particular embodiment,the new transmission frequency can be determined (830) by using theestimated distance to look up the new transmission frequency value froma table stored in a memory of the external module 110. The new value isset to a frequency which has a predetermined difference from theresonant frequency of the system.

In one embodiment, the system operates with hysteresis in order to avoidfrequent adjustments to the transmission frequency which might lead toinstability. As indicated at step 840, a new value of the transmissionfrequency obtained by table look-up is compared to the currenttransmission frequency value to determine whether it is greater than athreshold amount. If the difference is greater than the threshold, thetransmission frequency is set to the new value. (850) Operation thencontinues as before from step 810. However, if the difference betweenthe new and current values is not greater than the threshold, thetransmission frequency is not adjusted. Operation then continues asbefore from step 810.

FIGS. 9A-9C illustrate operation in accordance with a variation of theembodiment described above with respect to FIG. 7. In this embodiment, acondition within the implanted module 120, e.g., power demand, ismonitored relatively slowly, for example, just a few times each secondor perhaps at intervals of one second or greater. For example, FIG. 9Aillustrates measurements 910, 912, 914 of the power demand duringsuccessive intervals. In such case, the monitoring is insufficient tokeep up with all changes in power demand during the cardiac cycle of thepatient. Based upon the power measurements, a long-term power trend 916can be estimated.

FIG. 9B illustrates a predictive signal 920. The predictive signal is atime-varying representative of a predicted change in power demand due tothe cardiac cycle of the patient. The predictive signal can be based,for example, on a continuously obtained electrocardiogram (“EKG”)reading taken of the patient by an appropriate device within theexternal module 110.

Based upon the long-term power trend 916 and the predictive signal 920,the microcontroller 212 can adjust the instantaneous amount of power tobe supplied to the transmitting primary coil 114 such that a rapidlyvarying amount of power is transmitted as represented by curve 930 (FIG.9C). The rapidly varying power curve 930 reflects both the long-termtrend line 916 (shown in dashed form in FIG. 9C) and the rapidly-varyingpredictive signal.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

For example, in an implanted module 120 in accordance with a variationof the above-described embodiment (FIG. 3), the backup telemetry module402 may include a radio frequency (RF) transmitter for transmittingsignals to the external module 110 (FIG. 3). In this case, the backuptelemetry module 402 can operate without having a coil (e.g., such ascoil 406; FIG. 4) being inductively coupled with a primary coil 114 inthe external module 110. In such variation, the RF transmitter of thebackup telemetry module 402 can be designed to transmit signals over afrequency range different from that used by the primary RF transmitter224 and avoid interference which affects the primary RF transmitter 224.

1. (canceled)
 2. A circulatory assist device comprising: (a) a pumpadapted for implant in a patient, the pump having an electric motor, thepump having a time-varying power demand which is adapted to vary with acardiac cycle of the patient; (b) an implantable power unit adapted formounting within a body of the patient, the implantable power unit havinga secondary coil, a power circuit connected to the secondary coil forsupplying power to the pump so that power applied to the secondary coilcan be transmitted to the pump, and a monitoring circuit operable tomonitor a condition of the power circuit and to send a transcutaneoustelemetry signal representing the monitored condition, (c) an externalpower supply having a primary coil adapted for transcutaneous inductivecoupling with the secondary coil, a drive circuit operable to apply analternating current to the primary coil, and a control circuit operableto receive the telemetry signal and to adjust the alternating current inthe primary coil at least in part in response to the telemetry signal,the monitoring circuit and control circuit being operable to monitor thecondition of the power circuit and to vary an amount of the powerapplied to the secondary coil in accordance with the time-varying powerdemand of the pump in intervals of less than one second of a pluralityof periodic intervals during the cardiac cycle, wherein the controlcircuit is operable to adjust the alternating current in the primarycoil at a sufficient rate such that the implantable power unit cansupply power to the pump without drawing power from an implantedbattery.
 3. A device as claimed in claim 2, wherein the monitoring andcontrol circuit are operable to monitor the condition of the powercircuit in response to a change in coupling between the primary andsecondary coils and adjust the alternating current in the primary coilso as to vary the alternating current in the primary coil substantiallyin accordance with the change in coupling and maintain the monitoredcondition within the desired range despite the change in coupling.
 4. Adevice as claimed in claim 2, wherein the monitoring circuit is arrangedto monitor the condition of the power circuit and to send the telemetrysignal at least 50 times per second.
 5. A device as claimed in claim 2,wherein the monitored condition is a voltage at the secondary coil.
 6. Adevice as claimed in claim 2, wherein the power circuit further includesa battery and a routing circuit arranged to route power from thesecondary coil to the motor, to the battery or both when power isreceived through the secondary coil and from the battery to the motorwhen power is not received by the secondary coil, and wherein themonitoring circuit and control circuit are operable to vary the currentin the primary coil so as to maintain the condition within the desiredrange during the charging operation and during operation when thebattery is fully charged.
 7. A device as claimed in claim 2, wherein themonitoring circuit further includes a temperature sensor operable todetect a temperature within the implantable power unit.
 8. A device asclaimed in claim 7, wherein the monitoring circuit is arranged to reducea supply of current by the power circuit to at least one of the pump oran implanted battery coupled to the implantable power unit when thedetected temperature exceeds a threshold.
 9. A device as claimed inclaim 2, wherein the control circuit is operable to automatically adjustthe first frequency to adjust a resonance between the primary andsecondary coils for efficient and stable transmission of power betweenthe primary and secondary coils.
 10. A device as claimed in claim 2,wherein the monitoring circuit is operable to send, and the controlcircuit is operable to receive the telemetry signal in each of aplurality of periodic intervals of less than one second and at asufficient rate such that the implantable power unit can operate withoutan implanted battery for supplying power to the pump.
 11. A circulatoryassist device comprising: (a) an implantable pump adapted for mountingwithin the body of the patient, the pump having an electric motor with atime-varying power demand adapted to vary with a cardiac cycle of thepatient; (b) an implantable power unit adapted for mounting within thebody of the patient, the implantable unit having a secondary coil, apower circuit connected to the secondary coil for supplying power to thepump so that power applied to the secondary coil can be transmitted tothe pump, and a monitoring circuit operable to monitor a condition ofthe power circuit and to send a transcutaneous telemetry signalrepresenting the monitored condition, (c) an external power supplyhaving a primary coil adapted for transcutaneous inductive coupling withthe secondary coil, a drive circuit operable to apply an alternatingcurrent to the primary coil, and a control circuit operable to receivethe telemetry signal and to adjust the alternating current in theprimary coil at least in part in response to the telemetry signal,wherein the control circuit is operable to adjust the alternatingcurrent in the primary coil in response to the telemetry signal so as tomaintain the monitored condition in the power circuit within a desiredrange in each of a plurality of intervals during the cardiac cycle, eachinterval having a duration of less than one second, wherein the controlcircuit is operable to adjust the alternating current at a sufficientrate such that the implantable power unit can supply power to the pumpwithout drawing power from an implanted battery.
 12. A device as claimedin claim 11, wherein the monitoring and control circuit are operable tomonitor the condition of the power circuit in response to a change incoupling between the primary and secondary coils and adjust thealternating current in the primary coil so as to vary the alternatingcurrent in the primary coil substantially in accordance with the changein coupling and maintain the monitored condition within the desiredrange despite the change in coupling.
 13. A device as claimed in claim11, wherein the monitoring circuit is arranged to monitor the conditionof the power circuit and to send the telemetry signal at least 50 timesper second.
 14. A device as claimed in claim 11, wherein the monitoredcondition is a voltage at the secondary coil.
 15. A device as claimed inclaim 11, wherein the power circuit further includes a battery and arouting circuit arranged to route power from the secondary coil to themotor, to the battery or both when power is received through thesecondary coil and from the battery to the motor when power is notreceived by the secondary coil, and wherein the monitoring circuit andcontrol circuit are operable to vary the current in the primary coil soas to maintain the condition within the desired range during thecharging operation and during operation when the battery is fullycharged.
 16. A device as claimed in claim 11, wherein the monitoringcircuit further includes a temperature sensor operable to detect atemperature within the implantable power unit.
 17. A device as claimedin claim 16, wherein the monitoring circuit is arranged to reduce asupply of current by the power circuit to at least one of the pump or animplanted battery coupled to the implantable power unit when thedetected temperature exceeds a threshold.
 18. A device as claimed inclaim 11, wherein the control circuit is operable to automaticallyadjust the first frequency to adjust a resonance between the primary andsecondary coils for efficient and stable transmission of power betweenthe primary and secondary coils.
 19. A device as claimed in claim 11,wherein the monitoring circuit is operable to send, and the controlcircuit is operable to receive the telemetry signal in each of aplurality of periodic intervals of less than one second and at asufficient rate such that the implantable power unit can operate withoutan implanted battery for supplying power to the pump.